Parachutes are the lightest, most cost-effective device for decelerating a vehicle in the atmosphere. They are also very reliable when manufactured according to strict quality control procedures, maintained properly and used within their design operating envelope. The very dynamic nature of parachuting activities has created the necessary high standards of engineering associated with them.
Present day parachutes have many applications, including sport parachuting, airdrop of troops and supplies, emergency aircrew escape, stabilization of ordnance, and recovery of many other aerospace vehicles. Their configurations range from round, non-steerable, hemispherical shaped parachutes that simply create drag and slow descent to wing shaped gliding parachutes that offer precision control and maneuverability. Round parachutes are used for many applications because their purpose is simple, to slow a payload to a descent speed that is conducive to a safe touchdown. It is this simplicity that enhances their reliability.
Parachutes have traditionally been fabricated from woven textiles in the form of fabrics, tapes, webbing, and thread. The technology of round parachutes has changed very little since the advent of modern aviation at the turn of the century. The major innovations over this time have been in the raw fiber materials and the weaving, braiding and coating processes used to fabricate the woven textiles.
Silk and cotton, organic materials that are extremely susceptible to degradation, were commonly used in parachutes before World War II. Most silk parachutes were limited to life cycles of less than seven years and required numerous periodic inspections. Nylon, introduced by DuPont in 1939, was probably the most significant contribution to modern parachute technology. Another significant material enhancement was the introduction of Kevlar in 1979. Nylon and Kevlar are the primary materials used in the fabrication of modern parachute textiles.
The basic structure of a round parachute consists of the canopy and suspension lines. The canopy, which creates the aerodynamic drag necessary to decelerate the payload, can have a variety of shapes. Examples of round canopy shapes include flat circular, hemispherical, semi-hemispherical, conical, and multi-conical. These are only a few of the more common options for canopy shape. The designer can choose any shape that will meet the aerodynamic requirements of the specific application.
Round parachutes commonly have a vent at the apex of the canopy to allow a small amount of airflow to escape which provides stability during the parachute's descent. The suspension lines are attached to the skirt, i.e. the lower perimeter, of the canopy. The opposite end of the suspension lines are connected to the payload, either directly or via a riser or series of risers.
The most common method of fabricating a canopy is by sewing together a series of trapezoidal shaped fabric panels, called gores, to form its desired shape. The strength of this basic canopy structure is dependent on the strength of the fabric and of the sewn seams. Thea canopy can be structurally enhanced by sewing a series of reinforcement tapes to its top surface at critical locations. Radial bands are sewn to the vertical seams of the canopy gores from the skirt to the vent. Lateral bands are sewn horizontally to the canopy around its entire circumference at the skirt band, at the vent band, and possibly at one or more intermediate locations between the skirt and the vent.
The suspension lines are attached to the radial bands at the canopy skirt and converge to a riser or set of risers at the opposite end. The riser connects the parachute to its payload. Vent lines are attached to the radial bands and routed across the vent opening to an adjacent radial band on the opposite side of the vent.
One of the primary considerations in parachute design is strength-to-weight ratio. The structure has to be strong enough to sustain the aerodynamic forces associated with decelerating its payload, yet be lightweight and unobtrusive so as to maximize the utility of the aircraft associated with their operation.
One such attempt at increasing the strength-to-weight ratio of a round parachute has been through the use of reinforced thin films. The properties of thin polymer films can be enhanced by laminating fibers to them to create a reinforced thin film. These "composite" structures combine the physical properties of the individual materials to optimize their overall properties. The thin film is a flexible, low-permeability membrane that provides the aerodynamic characteristics necessary for these devices. The fiber reinforcements provide structural integrity and act as a barrier to tear propagation.
Reinforced thin films have been used with a moderate degree of success in the manufacturing of sails and balloons. Rolls of extruded thin films, in thicknesses ranging from a quarter mil to several mils, are laminated with a pattern of fibers, either individual fibers or fibers provided in a pre-woven scrim form. Sections of this material are then cut into shapes that can be assembled into the required sail or balloon shape in the same way that three dimensional parachute canopy shapes are made from smaller gores. However, instead of sewing, the sections are bonded together with an adhesive.
Since the fibers provide the primary structural support in these reinforced thin film membranes, an effective bonded seam must transfer loads from the fibers in one section to those in an adjacent section. To attain maximum seam effectiveness, the fibers, which are typically spaced from 1/8 inch to one inch apart, would have to be aligned in a one-to-one pattern along the entire length of the seam. Otherwise, the seam strength is primarily a function of the thin film strength, which is considerably less than that of the fiber reinforcements. However, it is very difficult and thus quite time consuming and expensive to obtain the necessary fiber alignment to provide sufficient strength along the seams.
Bonded seams have therefore been the primary weak link in the development of high performance reinforced thin film structures. This problem is especially detrimental to parachute structures which can be subjected to the very high structural loads that are encountered when decelerating a vehicle or other payload from high speeds. Although modern adhesives are very effective, problems are encountered in designing a seam that can effectively transfer structural loads across the fibers in adjacent gores.
One sail manufacturer has created a method for eliminating these structural discontinuities associated with bonded seams. Raw thin film is cut into the geometric sections required to form the desired overall sail shape and laid onto a large mold. Then, using a computer controlled six degree-of-freedom gantry that scans the mold surface, the reinforcement fibers are then bonded to the thin film in a continuous pattern across the entire sail. This fiber pattern is designed to accommodate the stress requirements for each specific sail shape. However, this method of manufacture would be cost prohibitive for parachute applications because of the facility requirements for accommodating such a large mold and the equipment necessary to lay out complex fiber patterns.
In summary, a method for cost effectively producing a reinforced thin film parachute that eliminates the need for bonded structural seams would provide significant improvements over current parachute technology.